Fix typo in alternate psuedo-code

Author: Michael Clark

opcode-pseudocode-alt

@@ -28,7 +28,7 @@ slli       "rd ← ux(rs1) « ux(imm)"
 srli       "rd ← ux(rs1) » ux(imm)"
 srai       "rd ← sx(rs1) » ux(imm)"
 add        "rd ← sx(rs1) + sx(rs2)"
-sub        "rd ← sx(rs1) + sx(rs2)"       # rs1 + ¬rs2 + 1
+sub        "rd ← sx(rs1) - sx(rs2)"       # rs1 + ¬rs2 + 1
 sll        "rd ← ux(rs1) « rs2"
 slt        "rd ← sx(rs1) < sx(rs2)"
 sltu       "rd ← ux(rs1) < ux(rs2)"